1. Field of the Invention
This invention relates to a developing roller used in contact with an image-bearing member (photosensitive drum) set in an image forming apparatus employing an electrophotographic system, such as a copying machine, a printer or a facsimile receiving set, and also to a developing assembly, a process cartridge and an electrophotographic image forming apparatus which use the same.
2. Description of the Related Art
It is substantially essential for the developing roller to be provided with an elastic-material layer containing a rubber component or a resin component, in order to secure the nip width between the elastic-material layer and the photosensitive drum. In order that any low-molecular component which may ooze out of such an elastic-material layer can be kept from adhering to the photosensitive drum, a configuration is employed such that a cover layer is provided on the elastic-material layer.
Asker-C hardness of the developing roller is closely concerned with deterioration of toner with time. More specifically, too high Asker-C hardness may accelerate the deterioration of toner with time. Hence, it has conventionally been proposed that the Asker-C hardness of the developing roller is set to be in the range of from 25° or more to 85° or less [see, e.g., Japanese Patent Applications Laid-open No. 2001-166533 (Patent Document 1) and No. 2005-121728 (Patent Document 2)].
As another problem in the developing roller in which the presence of the elastic-material layer is essential as stated above, there is partial permanent set that comes about when a contact member such as a photosensitive drum and a cleaning blade is kept in contact with one another over a long period of time. When electrographic images are formed by using a developing roller in which such partial permanent set has occurred, faulty images may be brought about correspondingly to the part where the permanent set has come about.
Thus, it has hitherto been noted as a problem to be resolved that a developing roller in which the permanent set cannot easily be caused while having a low hardness. For example, Japanese Patent Applications Laid-open No. 2006-106323 (Patent Document 3) and No. 2005-248084 (Patent Document 4) disclose developing rollers made up variously so as to resolve such a problem. However, according to studies made by the present inventors, it can not necessarily be said that conventionally proposed developing rollers have been made sufficiently effective in resolving such a problem, and they have come to realize that a novel developing roller should be created which can resolve the problem at a higher level.